4/15 STABILITY 



current affects the grid-potential. The effect represented by the 

 lower arrow is determined by the valve-designer, that of the 

 upper by the circuit-designer. 



Such systems whose variables affect one another in one or more 

 circuits possess what the radio-engineer calls ' feedback ' ; they are 

 also sometimes described as ' servo-mechanisms '. They are at 

 least as old as the Watt's governor and may be older. But only 

 during the last decade has it been realised that the possession of 

 feedback gives a machine potentialities that are not available to a 

 machine lacking it. The development occurred mainly during the 

 last war, stimulated by the demand for automatic methods of 

 control of searchlight, anti-aircraft guns, rockets, and torpedoes, 

 and facilitated by the great advances that had occurred in elec- 

 tronics. As a result, a host of new machines appeared which 

 acted with powers of self- adjustment and correction never before 

 achieved. Some of their main properties will be described in 

 S. 4/16. 



The nature, degree, and polarity of the feedback usually have 

 a decisive effect on the stability or instability of the system. In 

 the Watt's governor or in the thermostat, for instance, the con- 

 nexion of a part in reversed position, reversing the polarity of 

 action of one component on the next, may, and probably will, 

 turn the system from stable to unstable. In the reaction circuit 

 of the radio set, the stability or instability is determined by the 

 quantitative relation between the two effects. 



Instability in such systems is shown by the development of a 

 ' runaway '. The least disturbance is magnified by its passage 

 round the circuit so that it is incessantly built up into a larger 

 and larger deviation from the central state. The phenomenon is 

 identical with that referred to as a ' vicious circle '. 



4/15. The examples shown have only a simple circuit. But more 

 complex systems may have many interlacing circuits. If, for 

 instance, as in S. 8/2, four variables all act on each other, the 

 diagram of immediate effects would be that shown in Figure 



4-±=*3 3- 4 



2 

 C 



Figure 4/15/1. 



